Tensioners are widely used to suppress vibration generated during the operation of a timing belt or timing chain, which transmits rotation between the crankshaft and camshaft of an internal combustion engine. The tensioner is used not only to suppresses vibration, but also to maintain proper tension in the belt or chain. In a typical tensioner configuration, a plunger, which protrudes from a tensioner housing, presses on one side of a tensioner lever near an end of the lever which is pivotally mounted on an engine housing. A shoe surface, on the opposite side of the lever, is held in sliding contact with, and maintains tension, in the slack side of the chain, which is the side moving toward the driven sprocket.
FIGS. 10 and 11 show a conventional ratchet-type hydraulic tensioner 50, which includes a ratchet locking mechanism 53. This conventional tensioner is disclosed in the specification of Japanese patent application No. 013068/2000. This tensioner comprises a plunger 51 slidable in, and protruding from, a housing 52, and biased in the protruding direction by a spring 56. A ratchet pawl 53a is pivotally supported on the tensioner housing 52. The ratchet pawl is accommodated in a recess constituted by a groove 52e formed between side walls 52d of the housing 52 and is biased by a ratchet pawl spring 53c in such a way that the ratchet mechanism blocks retracting movement of the plunger 51 by causing teeth 53d of the ratchet pawl 53a to engage ratchet teeth 51a provided on the outer periphery of the plunger 51 on one side thereof.
The tensioner 50 further includes a hydraulic check valve mechanism 55, through which oil, supplied under high pressure by an oil pump activated by the engine, is fed to a high pressure oil chamber 54 inside the plunger 51.
In the ratchet-type hydraulic tensioner 50, the teeth 53d of the pawl engage the ratchet teeth 51a. When the plunger 51 receives an impact force from the tensioner lever, the plunger 51 moves, in the retracting direction, against the biasing force exerted by spring 56. A limited amount of retracting movement is permitted, within the range of backlash of the ratchet teeth 51a and the pawl teeth 53d. Retracting movement of the plunger causes the hydraulic valve mechanism 55 to close. Then, oil in the high-pressure oil chamber 54 in the plunger 51 absorbs the impact force from the tensioner lever by leaking through the clearance between the outer peripheral surface of the plunger 51 and the inner peripheral surface of the plunger-receiving hole.
Although not illustrated, when rate at which oil is supplied to the tensioner 50 by an oil pump increases rapidly, an excessive amount of oil can be fed to the high-pressure oil chamber 54, causing the plunger 51 to protrude farther than necessary. In particular, when a cold engine is started, the oil supplied by the engine oil pump has a high viscosity. Because of the high viscosity, the oil pressure in the high-pressure oil chamber 54 can be higher than usual, and, as a result, the plunger 51 can be pushed outwardly toward the tensioner lever farther than necessary. In the case of the conventional ratchet-type hydraulic tensioner 50, engagement of the pawl teeth with the ratchet teeth prevents the plunger from returning easily into the tensioner housing 52.
As a result of excess pressure and high viscosity of the oil, the plunger may exert excessive pressure on the tensioner lever, resulting in abnormally high tension in a timing chain. This has resulted in problems, including the generation of a whistling sound and premature breakage of the timing chain.
Accordingly, objects of the invention are to solve the above-mentioned problems encountered in the operation of conventional ratchet-type hydraulic tensioners, and to provide a ratchet-type hydraulic tensioner in which, when excessive oil is supplied by an oil pressure generating source such as an oil pump or the like, the excess oil, in an oil supply passage leading from an oil supply opening to an hydraulic valve mechanism, is discharged to the exterior of the tensioner in order to prevent an excessive amount from being supplied to the plunger. In this way, abnormal protrusion of the plunger is prevented, the whistling sound of the chain is reduced, proper tension is maintained in the chain, so that the chain travels reliably and smoothly, and the durability of the tensioner, and the chain and other components with which the tensioner cooperates, is improved.